In a vehicle such as an automobile, electrical loads such as lamps or motors are usually installed at low-side, since a chassis of the vehicle has a ground potential in general. Then, high-side switches, in which the loads are connected between the switches and the ground, are often used, and also N-channel MOSFETs that operate as source followers are often used as switch elements due to their low on-resistance characteristics and low cost.
It is necessary to apply a higher gate voltage than the power supply voltage to drive a high-side switch composed of the N-channel MOSFET that operates as a source follower. For this purpose, a step-up circuit is needed. A typical circuit for the step-up circuit is known as a charge pump circuit. An oscillation circuit is provided in the charge pump circuit to receive/transfer electric charges charged in capacitors.
Besides that, it is necessary to reduce incoming noises for electronic devices in general. For example, an incursion of noises to an AM band (531 to 1602 kHz) or traffic information service (1620 kHz) causes a problem to devices on-vehicle such as a radio receiver.
Therefore, a system to reduce incoming noises is required. For example, Patent Document 1 discloses a generation control device for a vehicle that reduces noises occurred from a charge pump circuit. The generation control device for a vehicle is formulated so as to provide a low frequency of an oscillation signal to drive the charge pump circuit under predetermined conditions. The predetermined conditions are determined as follows: when a magnetic field current detected by a magnetic field current detecting circuit is lower than a predetermined threshold of the magnetic field current, when a conducting time of a switching element detected by a conducting time detecting circuit is lower than a predetermined threshold of the conducting time, and when an induced voltage of a stator coil detected by an induced voltage detecting circuit is lower than a predetermined threshold of the induced voltage. In addition, a lowered frequency of 400 kHz of an oscillation signal is shown, as an exampled, compared to a rectangular wave of 800 kHz of initial oscillation frequency.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2005-312247A